Can anything stop the big pig invasion? | Science News for Students

Can anything stop the big pig invasion?

Scientists are scrambling to root out wild hogs, a destructive invasive species
Apr 19, 2018 — 6:45 am EST
feral pig

A feral pig looks similar to the domestic pig from which it has descended. Each year, these animals are blamed for causing large and amazingly costly damage in the United States alone.


The call came one morning in the spring of 2013. The cemetery was a mess.

Charlotte Watson remembers it clearly. She works in the courts in New York City. She also runs an organization that protects a historic cemetery in rural Texas, where she grew up. Named Willow Wild, this cemetery sits on 36 hectares (90 acres) in Bonham. The site is about 130 kilometers (80 miles) northeast of Dallas. Someone in Bonham who regularly visited the cemetery was the first on the scene.

“Something terrible had happened,” Watson recalls — wild pigs!

pig damage
Overnight, wild pigs can destroy a lawn, garden or field of crops.
USDA APHIS photo by Scott Woodruff/Flickr (CC BY 2.0)

They had barged in and uprooted wide patches of grass. It looked like someone had ripped out the grass and tilled the soil. No grave markers were knocked over, but “it looked really bad,” says Watson. “You couldn’t imagine [the grass] would grow back.”

For the next few weeks, wild pigs slept under the surrounding trees by day and slipped into the cemetery by night: They came to root in the soil for grubs. These thick white worms, which would grow up to become beetles, live several centimeters (a few inches) below the soil surface.

The invaders weren’t going to leave quickly on their own. Watson and her group had to face some tough questions about how to deal with these far-from-benign swine.

Texas is hardly alone in facing marauding pigs. These wild swine can be found in nearly every U.S. state. They've also been spotted in Canada, and many cross the border from Texas into Mexico. In the United States, they have become concentrated in southeastern states. They also wreak havoc in other countries, including the United Kingdom and Australia. In Germany, hordes of pigs dig up gardens in the suburbs of Berlin.

pigs in cornfield
Wild pigs each year feast on crops and cause $1.5 billion in damage in the United States alone.
USDA APHIS photo Craig Hicks/Flickr (CC BY 2.0)

Wild pigs cause some $1.5 billion in damage every year in the United States, mostly to crops, according to the U.S. Department of Agriculture (USDA). They also pose a health hazard. Wild pigs carry at least 30 diseases and 37 parasites (organisms that live and feed on a living host). Some of these diseases and parasites can spread to other animals. They can also infect people who eat or breathe the germs. And when cornered, wild pigs can, though rarely, attack people. Last December, for instance, a feral pig attacked the German hunter who had shot it. The man would later die.

Wildlife biologists around the world want to understand these feral swine to halt the menace. They’re tracking the animals to understand their behavior and predict where they’ll go. Researchers are testing new traps, including some that send real-time video to smartphone apps.

Stopping the pigs is difficult, in part, because they’re canny. “They’re one of the smartest animals on the planet,” notes wildlife biologist Alan Leary. He works for the Missouri Department of Conservation in the state's capital, Jefferson City. “We have to continually come up with new techniques to stay ahead of them,” he says.

Right now, the pigs are winning.

Swine invasion

They go by many names: wild pigs, wild hogs, feral swine, feral pigs and wild boars. But they’re all Sus scrofa, a pig species native to Europe, Asia and North Africa.

A group of wild pigs can devastate corn or soybean fields overnight. The swine can shred riverbanks and wreak havoc near cities, even in people’s yards. They destroy landscaping. The muddy mess they leave behind often looks like the crater from a bomb.   

In the last few decades, the pig menace has worsened in the United States because the animals don't have any natural predators. What’s more, people haven't found an effective way to stop them. In the first week after the fastest highway in the United States opened — south of Austin, Texas — three cars collided with wild pigs. And then there was that F-16 fighter jet, back in 1988, that collided with feral pigs on a Florida runway. The pilot ejected to safety. His $16 million jet? Destroyed.

aerial pig herd
This herd of wild pigs is roaming through Australia.

There’s a term to describe critters like wild pigs: invasive species. These organisms don’t cause problems in their natural habitats. But when people have introduced them into a new environment, either on purpose or by accident, they tend to cause problems. Sometimes big problems. Invading plants and animals can quickly gobble up available resources and make it harder for other species to thrive.

Invasives might outcompete native species, causing the natives to decline. Or the invasive species could damage crops and natural areas, such as woodlands. Invasive insects might kill trees, leaving a forest more likely to burn. One 2005 study estimated that invasive species cause $120 billion in U.S. damage each year.

Pigs are not native to North America. Spanish settlers who colonized Florida in the 16th century brought along swine. For the first couple hundred years, populations of these animals stayed small and contained. They rarely roamed beyond the Florida panhandle.

Then hunters became interested in wild pigs toward the end of the 20th century and everything changed.

“Their popularity spawned hundreds of commercial fenced operations of wild boar hunts,” says Jack Mayer. He’s a wildlife biologist at the Savannah River National Laboratory in Aiken, S.C., and has been studying wild pigs for more than 40 years. Ranchers and farmers began to keep wild pigs for hunters. Alas, he says, the animals couldn’t be contained. “Virtually every state has some of those operations.” Now, he says, “At least one or more of those operations in each state is leaking pigs.”

And their wild populations have exploded in the past 20 years. Partly that’s because pigs can live anywhere, eat just about anything — from acorns to small animals — and reproduce quickly. They can adapt to almost any climate. Mayer says they’ve been spotted in 48 U.S. states (including Hawaii and Alaska). These wild swine have established populations in 36. For now, only Wyoming and Rhode Island appear to be free of feral pigs, says Mayer.

trapped swine
Feral pigs ramble freely in most U.S. states. Traps may help control where they roam.
USDA APHIS photo by Clint Turnage/Flickr (CC BY 2.0)

Leary, in Missouri, says people can be part of the problem. Maps show pig populations separated from each other by hundreds of kilometers (miles). The pigs probably didn’t hoof it all that way. People must have transported them. “We know that pigs don’t fly, and they had to get there somehow,” he says. Some people intentionally release wild pigs into an area to create a hunting ground, even though it’s illegal. Such actions give rise to new pig populations.

The problem isn’t going away. The Texas Department of Agriculture predicts that if nothing is done, the pig population in that state will triple within five years. A federal program, the National Feral Swine Damage Management Program, has been created to curb the invasive species’ expansion. Already, it estimates, the United States host some 5 million or 6 million feral pigs. And their numbers are growing.

Indeed, that growth shows no signs of slowing down, according to a 2017 study in the Journal of Applied Ecology. USDA researchers studied pig populations from 1992 to 2012. If they continue to spread at the same rate, it estimates that most counties across the United States will be plagued by wild pigs within 30 to 50 years. 

Hunting — sometimes even from helicopters

Wild pigs can run as fast as 48 kilometers (30 miles) per hour and scamper over fences a meter (three feet) high. These swine can reproduce once or twice every year, and a typical litter includes five or six piglets. (Some people in the South even joke that “pigs are born pregnant.”) A single pig may grow to weigh hundreds of kilograms (pounds).

Scientists have a lot of information about the habits and behaviors of wild pigs, says Mark Smith. He’s a wildlife biologist at Auburn University in Alabama. “Everybody’s staring at the same science,” he says. “Our role is to get the best information out there, see it and make good judgments off good science.”

Some scientists have run computer models of pig populations. Then they analyze what control tactics might prove most likely to bring those populations down. To completely rid an area of pigs, more than two-thirds of the animals have to be removed every year, those computer models suggest. And that removal rate would need to be continued year after year after year — until there were no more pigs.

How might that removal be accomplished? These are, after all, wily animals.

Some states have established hunting seasons. Others have brought in sharpshooters, or trained hunters. Others offer rewards for feral-pig carcasses. Texas passed a law in 2011 that allowed people to shoot the pigs from helicopters. Now some people pay thousands of dollars for the experience.

Beyond hunting

Smith doubts that hunting will ever solve the problem. Most hunters stop after they get one or two pigs. What’s more, some scientists have observed that pigs can learn from the hunts. They may adapt their behaviors to avoid hunters. Some might move away from sites where people prefer to hunt. Or the animals might eat at night, instead of by day. That could make them harder to find. Hunting and sharpshooting will likely only work for the last few pigs of a sounder. (Sounder is the name for a group of wild pigs.)

Leary says trapping offers the best chance of catching the most pigs. But the traps have to be smarter than the animals.

sensor camera
A motion sensor camera took this night time image of wild pigs caught in a trap in Tennessee.
Rodney Woodson

Pigs can climb, so the traps must be tall and not have sharp corners that can give a pig a hoofhold to climb out. And traps have to be able to catch all of the pigs in a sounder. If any get away, they’ll know enough to not return to this trap site. Then, unless they’re tracked down some other way, these pigs may colonize a new area. 

Newer traps incorporate new technologies. Some include motion-sensor cameras that connect to smartphone apps. The cameras watch the trap, which looks like a big ring of tall metal fencing. There are one or two open gates to the enclosure. When pigs arrive, the camera alerts the landowner or ranger. Then, someone can watch the scene in real-time, from wherever they are. Once all the pigs have wandered into the fenced pen, the trapper can drop a gate through the app with a swipe of a finger.

It’s not cheap, though. A basic trap will cost a farmer hundreds of dollars. With the sensors, cameras and app, that cost can climb into the thousands.

Traps also won’t be able to get all the pigs, says Mayer. So scientists are looking at other approaches. Biologists in Alabama and Colorado are studying possible poisons. But there’s no guarantee that only a pig will consume it. Texas, for example, has black bears. They will eat almost anything that pigs eat. Livestock also might take the bait. Researchers will have to figure out how to poison wild pigs without harming bears or other animals.

bait trap
Meet the HOG-HOPPER. Animal-control experts in the United States and Australia are developing devices like this one. They use bait to draw wild pigs into a trap.
USDA APHIS photo Tyler Campbell/Flickr (CC BY 2.0)

At Auburn, Smith says veterinarians are also working on pig birth-control strategies. These are drugs or devices to prevent reproduction. Researchers have developed such drugs that work. But here's the snag: Someone would have to inject it directly into each pig. And that isn't practical for wild animals, which could be anywhere — and hiding. 

Such efforts to get rid of pigs have the best chance of working where the animals are new, say experts. But the challenge of removing every pig, permanently, is daunting. So scientists want to focus their efforts on reducing pig populations and limiting the damage they cause.

Smith says the way to reduce and control the wild-pig problem will take a combination of methods. First, though, people have to be convinced that their moving and releasing pigs is a serious problem. Traps may then be useful to get most of the pigs. Birth control or poisons, if they don’t cause extensive harm, may help. And sharpshooters may be able to get the last few. “Those last pigs are where you’re spending all your money,” says Smith.

Charlotte Watson, at the cemetery in Texas, went through her own ordeal to get rid of the pigs. First, she hired someone to set up traps. “Ideally, the pigs run in there and they can’t get back out,” she says. Then a trapper would come and get the pigs. The cemetery would pay for every animal caught.

Except that it didn’t work.

“They didn’t pay any attention to the traps,” she says of the pigs. “Of course, hogs are very smart.” A few weeks later, though, the pigs moved to another neighborhood. They haven’t returned. Though Willow Wild may have been spared for now, there’s no guarantee the swine won’t be back wreaking havoc once more.

Correction: The text has been adapted to note that explorers and settlers did not carry pigs to North America until the 16th century.

Power Words

(more about Power Words)

agriculture     The growth of plants, animals or fungi for human needs, including food, fuel, chemicals and medicine.

app     Short for application, or a computer program designed for a specific task.

beetle     An order of insects known as Coleoptera, containing at least 350,000 different species. Adults tend to have hard and/or horn-like “forewings” which covers the wings used for flight.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

biology     The study of living things. The scientists who study them are known as biologists.

boar     A term for the male of some mammals, including pigs and bears.

climate     The weather conditions that typically exist in one area, in general, or over a long period.

commercial     (in research and economics) An adjective for something that is ready for sale or already being sold. Commercial goods are those caught or produced for others, and not solely for personal consumption.

computer model     A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.

crop     (in agriculture) A type of plant grown intentionally grown and nurtured by farmers, such as corn, coffee or tomatoes. Or the term could apply to the part of the plant harvested and sold by farmers. 

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

environment     The sum of all of the things that exist around some organism or the process and the condition those things create. Environment may refer to the weather and ecosystem in which some animal lives, or, perhaps, the temperature and humidity (or even the placement of components in some electronics system or product).

federal     Of or related to a country’s national government (not to any state or local government within that nation). For instance, the National Science Foundation and National Institutes of Health are both agencies of the U.S. federal government.

feral     Animals that were once domesticated but now run wild. Examples may include feral dogs, horses or pigs.

forest     An area of land covered mostly with trees and other woody plants.

germ     Any one-celled microorganism, such as a bacterium or fungal species, or a virus particle. Some germs cause disease. Others can promote the health of more complex organisms, including birds and mammals. The health effects of most germs, however, remain unknown.

habitat     The area or natural environment in which an animal or plant normally lives, such as a desert, coral reef or freshwater lake. A habitat can be home to thousands of different species.

host      (in biology and medicine) The organism (or environment) in which some other thing resides. Humans may be a temporary host for food-poisoning germs or other infective agents.

information     (as opposed to data) Facts provided or trends learned about something or someone, often as a result of studying data.

invasive species     (also known as aliens) A species that is found living, and often thriving, in an ecosystem other than the one in which it evolved. Some invasive species were deliberately introduced to an environment, such as a prized flower, tree or shrub. Some entered an environment unintentionally, such as a fungus whose spores traveled between continents on the winds. Still others may have escaped from a controlled environment, such as an aquarium or laboratory, and begun growing in the wild. What all of these so-called invasives have in common is that their populations are becoming established in a new environment, often in the absence of natural factors that would control their spread. Invasive species can be plants, animals or disease-causing pathogens. Many have the potential to cause harm to wildlife, people or to a region’s economy.

journal     (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.

livestock     Animals raised for meat or dairy products, including cattle, sheep, goats, pigs, chickens and geese.

native     Associated with a particular location; native plants and animals have been found in a particular location since recorded history began. These species also tend to have developed within a region, occurring there naturally (not because they were planted or moved there by people). Most are particularly well adapted to their environment.

organism     Any living thing, from elephants and plants to bacteria and other types of single-celled life.

parasite     An organism that gets benefits from another species, called a host, but doesn’t provide that host any benefits. Classic examples of parasites include ticks, fleas and tapeworms.

population     (in biology) A group of individuals from the same species that lives in the same area.

predator     (adjective: predatory) A creature that preys on other animals for most or all of its food.

risk     The chance or mathematical likelihood that some bad thing might happen. For instance, exposure to radiation poses a risk of cancer. Or the hazard — or peril — itself. (For instance: Among cancer risks that the people faced were radiation and drinking water tainted with arsenic.)

sensor     A device that picks up information on physical or chemical conditions — such as temperature, barometric pressure, salinity, humidity, pH, light intensity or radiation — and stores or broadcasts that information. Scientists and engineers often rely on sensors to inform them of conditions that may change over time or that exist far from where a researcher can measure them directly.

simulation     (v. simulate) An analysis, often made using a computer, of some conditions, functions or appearance of a physical system. A computer program would do this by using mathematical operations that can describe the system and how it might change over time or in response to different anticipated situations.

smartphone     A cell (or mobile) phone that can perform a host of functions, including search for information on the internet.

species     A group of similar organisms capable of producing offspring that can survive and reproduce.

Texas     The second largest state in the United States, located along the southern border with Mexico. It is about 1,270 kilometers (790 miles) long and covers an area of 696,000 square kilometers (268,581 square miles).

United Kingdom     Land encompassing the four “countries” of England, Scotland, Wales and Northern Ireland. More than 80 percent of the United Kingdom’s inhabitants live in England. Many people — including U.K. residents — argue whether the United Kingdom is a country or instead a confederation of four separate countries. The United Nations and most foreign governments treat the United Kingdom as a single nation.

veterinarian     A doctor who studies or treats animals (not humans).


Journal: N.P. Snow et al. Interpreting and predicting the spread of invasive wild pigs. Journal of Applied Ecology. Vol. 54, December 2017, p. 2022. doi:10.1111/1365-2664.12866.

Journal: D. Pimentel et al. Update on the environmental and economic costs associated with alien-invasive species in the United States. Ecological Economics. Vol. 52, February 15, 2005, p. 273. doi:10.1016/j.ecolecon.2004.10.002.